The present invention relates to a novel fluorine-containing copolymer, an anti-reflection film, and a composition for forming a film that is usable for a coating and low dielectric constant materials useful as electric or electronic parts. Further, the present invention relates to an anti-reflection film having a low-refractive-index layer, formed by coating the composition, and it also relates to a polarizing plate and a display device (particularly a liquid crystal display device) equipped with the anti-reflection film.
Anti-reflection films are generally provided on the surface of optical products and the like, so that reflectance is reduced on the principle of optical interference, in order to prohibit reduction in contrast owing to reflection of external light, and also not to mirror the surrounding scene on the display surface. Particularly in such image display devices as a cathode-ray tube display device (CRT), a plasma display panel (PDP), and a liquid crystal display device (LCD), each of which requires that a display image be clearly viewed, the anti-reflection film is arranged at the outermost surface of the display.
Such anti-reflection films can be prepared by a method of forming a low-refractive-index layer, having an appropriate film thickness, on a high-refractive-index layer. The low-refractive-index material preferably has as low a refractive index as possible, from the viewpoint of anti-reflection property. At the same time, the low-refractive-index material, which is used at the outermost surface of the display, is required to have high scratch resistance. To achieve high scratch resistance in a thin film of thickness about 100 nm, adhesion to the lower layer and mechanical strength of the film are important factors. In addition, materials that can be used for a wet coating are preferable, from the viewpoint of productivity.
Introduction of a fluorine atom is generally used to reduce the refractive index of the material. For example, JP-A-8-92323 (xe2x80x9cJP-Axe2x80x9d means unexamined published Japanese patent application), JP-A-10-147749, JP-A-10-25388 and the like each describe that curable polymers, composed of an inexpensive fluorine-containing olefin as a main component, are used for anti-reflection films.
Curable fluorine-containing olefins, as a coating having weather resistance, have also been investigated for a long time. For example, many reports have been made in JP-A-57-34107, JP-A-61-258852, JP-A-61-275311, JP-A-62-85740, JP-A-62-292848, and the like.
Hexafluoropropylene, which is used in the Examples of the above-described publications, exhibits a lower crystallinity than tetrafluoroethylene. Therefore hexafluoropropylene is preferable as a material for use in a low-refractive-index layer of the anti-reflection film, from the viewpoints of not only the lower crystallization but also the refractive index and dissolution. As described in the Journal of Polymer Science, 4(6), 481 (1952), and Nippon Kagaku Kaishi (Journal of the Chemical Society of Japan), 51(1), 112 (1980), however, said monomer is sparingly homo-polymerizable in an ordinary solution radical polymerization reaction. In addition, even though hexafluoropropylene can be subjected to alternating copolymerization with a comparatively electron-donating monomer, such as vinyl ether, at most only about 50 mole % of said hexafluoropropylene can be incorporated in the polymer produced by alternating copolymerization. Therefore, there is a problem in accomplishing the purpose of sufficiently increasing the fluorine content to decrease the refractive index. On the other hand, it is possible to introduce 60 mole % or more of octafluoro-1-butene into a polymer, under specific conditions. However, there is also a problem that it is hard to incorporate 70 mole % or more of octafluoro-1-butene in a polymer, and the monomer is inferior for wide use.
The refractive index of a polymer can be reduced by reducing the component of a cross-linkable monomer having a high refractive index, while increasing the copolymerization component having a comparatively low refractive index such as ethyl vinyl ether. However, this method has a limit for use, and reduction in the density of the cross-linking group causes a disadvantage in scratch-resistance.
On the other hand, a method of increasing the fluorine content by introducing perfluoro(propyl vinyl ether) is described in the Examples of the above-described publications. However, because said monomer also exhibits the similar polymerization activity as hexafluoropropylene, it is difficult to incorporate, in a polymer, 50 mole % or more of the perfluoro(propyl vinyl ether) in a single use, or even in combination with hexafluoropropylene. Even though the fluorine content can be increased to some degree by introducing perfluoro(propyl vinyl ether), there are such problems that scratch-resistance of the thus-obtained film is remarkably reduced, and if the incorporated amount of the monomer increases, transparency of the film decreases.
Accordingly, there has been a need to develop materials for hexafluoropropylene-series copolymers that provide a high fluorine content, a low refractive index, both good dissolution in solvent and coating property, and both excellent transparency and scratch-resistance, without a reduction in the density of the cross-linking reactive group.
Further, JP-A-11-337706 describes use of fluorine-containing pre-polymers having an epoxy group, as anti-reflection films.
The hardened products of the above-described fluorine-containing olefin polymers exhibit unsatisfactory adhesion to a high-refractive-index layer, and therefore improvement in scratch resistance has been desired.
The present invention is a fluorine-containing copolymer represented by the following formula 1: 
wherein, in formula 1, Rf1 represents a fluorine-containing alkyl group having a straight chain, branched chain, or alicyclic structure having 1 to 30 carbon atoms, which may contain an ether bond; Rf2 represents a perfluoroalkyl group having 1 to 5 carbon atoms; A represents a constituent containing at least one reactive group that is capable of contributing to a cross-linking reaction; B represents an arbitrary constituent; a to d each represent a mole fraction (%) of each constituent, in which they satisfy the following conditions:
55xe2x89xa6a+bxe2x89xa695, 5xe2x89xa6axe2x89xa690, 5xe2x89xa6bxe2x89xa670, 5xe2x89xa6cxe2x89xa645, 0xe2x89xa6dxe2x89xa640 
Further, the present invention is a composition for forming a film containing the above-mentioned fluorine-containing copolymer.
Further, the present invention is an anti-reflection film having a low-refractive-index layer containing the above-mentioned fluorine-containing copolymer.
Further, the present invention is an anti-reflection film having the above-mentioned anti-reflection film on a transparent support.
Further, the present invention is a display device equipped with the above-mentioned anti-reflection film.
Other and further features and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.